Multiple Choice Questions

1.

Turner's syndrome chromosome pattern is:

XO

XXX

XYY

XXY

A.

XO

Turner's Syndrome is a chromosomal condition that mainly affects development in females. It is a monosomic condition with 44 + XO genotype. It occurs by the union of an abnormal egg and normal sperm or vice versa. These are sterile females with rudimentary ovaries, underdeveloped breasts, small uterus, short stature and mental retardation.

One of the genes present exclusively on the X-chromosome in humans is concerned with

baldness

red-green colour blindness

facial hair/moustaches in males

night blindness

B.

red-green colour blindness

The most common form of red-green colour blindness is an 'X chromosome linked recessive' disorder. The 'red' and 'green' genes are known to reside at the tip of the long arm of the X chromosome. Women have two copies of the X-chromosome, and so they may have normal colour vision, even if they carry one copy of the defective gene. Men have only one X-chromosome, and so will be colour blind.

In humans, sex of offspring is determined at the time of fertilization. Man is heterogametic i.e. having two types of gametes 22 + X and 22 + Y and is responsible for sex determination whereas female is homogametic i.e. having single type of gametes 22 + XX.

The main cause of Edward syndrome, Patau syndrome and Down syndrome is :

mutation of gene

change in both autosomes and heterosomes

change in autosomes

change in heterosomes

C.

change in autosomes

Down's syndrome is due to trisomy of 21st chromosome. Patau's syndrome is trisomy of 13th pair of chromosome and Edward's syndrome is due to trisomy of 18th chromosome. Therefore, these all syndromes are caused due to change in autosomes.

When paternal and maternal chromosomes change their materials with each other in cell division this event is called

synapsis

crossing over

bivalent forming

dyad- forming

B.

crossing over

Crossing over is the mutual exchange of segments of homologous chromatids during first meiotic prophase.

Synapsis is the fusion of chromosome pairs at the start of meiosis.

DNA replication precedes the start of Meiosis I. During Prophase I, homologous chromosomes pair and form synapses. The paired chromosomes are called bivalents and the formation of chiasmata caused by genetic recombination becomes apparent.

Dyad formation is a double chromosome resulting from the splitting of a tetrad during meiosis (germ cell formation).